Membrane process for recovery of carboxylic acids from dilute aqueous solutions

ABSTRACT

A C2 TO C4 MONO- OR DICARBOXYLIC ACID IS TRABSFERRED FROM A DILUTE AQUEOUS SOLUTION TO AN ORGANIC LIQUID THROUGH AN INTERVENING MICROPOROUS MEMBRANE. THE ORGANIC LIQUID COMPRISES AT LEAST ONE POLAR ORGANIC COMPOUND SELECTED FROM THE GROUP CONSISTING OF TRIALKYL PHOSPHIPHATES, DIALKYL ALKYL HOSPHONATES, ALKYL DIALKYL PHOSPHINATES, TRIALKYL PHOPHINE OXIDENS, DIALKYL ALICYCLIC AMIDOPHOSPHATES, DIALKYL SULFOXIDES AND TETRALKYL UREAS. AN IMPROVED METHOD FOR THE REMOVAL OF CARBOXYLIC ACID CONTAINANTS FROM AQUEOUS EFFUENTS OR RECYCLE STREAMS IS PROVIDED.

United States Patent O 3,808,267 MEMBRANE PROCESS FOR RECOVERY OF CAR- BOXYLIC ACIDS FROM DILUTE AQUEOUS SOLUTIONS James C. Davis, Benicia, and Robert R. Grinstead, Walnut Creek, Califi, assignors to The Dow Chemical Company, Midland, Mich. No Drawing. Filed Aug. 31, 1972, Ser. No. 285,545

Int. Cl. C01c 51/48 U.S. Cl. 260-527 R 4 Claims ABSTRACT OF THE DISCLOSURE BACKGROUND OF THE INVENTION It has been proposed to recover various carboxylic acids from aqueous solutions by direct contact with several different types of polar organic extractants. For example, see the following patents:

Patent Acid recovered Extractant U.S. 2,275,862 Acetic Cfi-Cm dialkyl ketones. U.S. 2,395,010 d Amyl acetate or methyl amyl ketone.

U.S. 2,446,231 do Ethyl disulfide. U.S. 2,480,380 Oyanoacetic Methyl isobutyl ketone. Czech. 137,177 Formic, acetic or Cyclohexanone.

C.A. 75: 63155y). propionic.

It has also been proposed to recover carboxylic acids from aqueous solutions by several membrane processes. For example, US. Pat. 2,411,239 suggests the use of tubular shaped semi-permeable, hydrophilic membranes for the separation of citric acid from aqueous solutions by dialysis. In a paper published in 1963 (Ind. Eng. Chem. Fundamentals, 2, pp. 53, 54) data is given for an experiment on reverse osmotic separation of acetic acid ctr-om a 0.25 molal solution in water, using a cellulose acetate membrane. At a transmembrane pressure difierential of 750 p.s.i.s., a 20.5% recovery of acetic acid was attained.

As a general proposition, the use of permeable membranes simply as artificial phase boundaries in extractive equilibration of liquid phases of difiering composition is known. For example, see Kirk-Othmer, Encyclopedia of Chemical Technology, 2nd Edition, vol. 7, p. 1. See also Chemical Abstracts, 74: 7106a.

The organic extractants which have been proposed for recovery of carboxylic acids from aqueous solutions are not efiicient enough for the processing of highly dilute solutions. In general, such solutions can be recycled or, if they do not also contain more toxic and/or persistent materials, can be discarded without constituting a serious source of pollution. However, this is not always the case. For example, the well known process for making phenol trom chlorobenzene produces a waste brine stream containing about 18-20 wt. percent of sodium chloride, about 0.2% sodium acetate and trace amounts of phenol, benzene and acetone. Such streams cannot be disposed of in surface waters and have too high a content of organics to be suitable as a feed to electrolytic chlor/alkali cells. It has now been found that such solutions can be made suitable as cell feeds at a minimal cost. The sodium ace- 3,808,267 Patented Apr. 30, 1974 tate content of the brine stream is converted to sodium chloride and acetic acid by addition of an equivalent amount of hydrochloric acid. The acetic acid is then removed by the practice of the instant invention.

In some cases, an otherwise suitable extraction process is impracticable due to phase separation difiiculties and/or losses of the extractant to the rafiinate phase. These problems are avoided by use of an appropriate membrane.

SUMMARY OF THE INVENTION Lower carboxylic acids are recovered from dilute aqueous solution by contacting the solution with one side of a microporous membrane while contacting the other side of the membrane with a liquid organic solvent for the acid or acids. The membrane has an average effective pore diameter of from 10 to 200 A. and is composed of a material which is not detrimentally elfected by either the aqueous or organic phases it is contacted with. The aqueous solution comprises a total of 2 weight percent or less of one or more two to tour carbon, monoor dicarboxylic acids optionally substituted with from one to three chlorines or fiuorines, or with one hydroxyl, oxo, cyano, methoxyl or ethoxyl group. The carbon to oxygen atomic ratio in each of said acids is not less than 1.0 nor greater than 2.0. The organic solvent comprises at least 1 weight percent of a polar extractant selected from the group consisting of (a) trialkyl phosphates n-o RO P- O wherein each R is a C to C alkyl group and the total number of carbons in the molecule is from 12 to 36, (b) dialkyl alkyl phosphonates wherein each R is a C to C alkyl group and the total number of carbons in the molecule is from 10 to 36, (0) alkyl dialkyl phosphinates alicyclic radical, I (cf) dialkyl sulfoxides wherein each R is a C to C alkyl group and the total number of carbons in the molecule is at least 8, (g) tetraalkyl ureas (RCHICHQ) zN-E-N (CHz-CH R"): wherein each R" is a C to C alkyl group and the total number of carbons in the molecule is from 12 to 36.

DETAILED DESCRIPTION OF THE INVENTION Representative lower carboxylic acids which may be recovered from aqueous solutions by the instant method are as follows:

Atomic ratio of carbon to Acid Formula oxygen Acetic CH3COOH 1. 0 Propanoic. CEHBCOOH 1. Butyric-. CZH'ICOOH 2. 0 Crotonic CHa-CH=CHCOOH 2. 0 Proplolic- CHEC-COOH 1. 5 Cyanoacetim- NC-CHz-COOH 1.5 Lactic GHBOH(OH)OOOH 1. 0 Glycolic; ethyl ether- CHs--OCH2CO OH 1. 33 Trifiuoroacetlc.. 1. 0 Pyruvic CODE 1. 0 Lactic; methyl ethe CH3CH(O CH3 00011 1. 33

Malonic; mono methyl 1. O

ester. ll

CHaO-CCHZCOOH Ethane-1,1-dicnrboxylic HOOCCH(CH@)COOH 1.0 Malelc or fumaric HOOCCH=GH-CO0H 1. 0

2,3-diohloroacry1lc ClCH=C-COOH 1. 5

Acetylene dicarboxylic acid- H0O 0-0 C-OOOH 1. 0 Methacrylic CH =C (CH3)-OOOH 2. 0

The aqueous solution of the acid or acids to be recovered may additionally comprise any other dissolved or suspended substances which do not prevent the requisite permeation through the membrane or otherwise interfere with the process. For example, other organic solutes, salts, suspended particles of carbon, ion-exchange resins or cellulosic fibers may be present. Whether or not other organics co-extract with the acid is immaterial to the instant invention. If co-extraction does occur and it is desired to separate the recovered acid, this can be done in a number of ways apparent to those skilled in the art. Since the primary utility of the present process is as a late stage clean-up step before disposal or recycle of an effluent, coextraction of any other organics will generally be advantageous.

A wide variety of membranes can be employed. The membrane structure may be isotropic or anisotropic and can be composed of any inorganic or organic substance which will not be detrimentally aifected by the liquids to be contacted and which can be formed into the requisite microporous structure. The latter term is not to be construed as limiting in terms of details of membrane structure and the term effective pore diameter as used herein does not restrict the process to the use of membranes pierced by discrete cylindrical capillaries. It is to be regarded, rather, as characterizing the requisite spacing between membrane molecules. At eifective pore diameters in excess of about 200 A., simple aggregated or convective flow of the aqueous solution in channels through the membrane tends to occur. At diameters of less than A., interaction between membrane molecules and the aqueous solution component molecules become large in comparison to interactions between solution molecules and transport will generally be unnecessarily restricted. Rejection of other solutes, such as salts, is due to insolubility in the organic phase, rather than by selective retardation within the membrane. The average effective pore diameter of this (molecular sieve) type of membrane can be deter. mined by electron microscopy or from the molecular dimensions of solute SpGGi WlliQh will and Will not Pa through the membrane. Since transport through such membranes is not diffusion limited, the membrane thickness is not a critical factor in the practice of the present process. The hydraulic resistance to flow will be proportional to the membrane thickness, but will generally be quite low in comparison to the diffusional resistance at the interface between the two liquid phases.

Membranes fabricated as bundles of elongated capillaries, i.e., as hollow fibers, afford maximal membrane areas and minimal membrane thicknesses, yet derive sufficient strength from their cross-sectional shape as to be self-supporting even under relatively high transmembrane pressure differentials. These advantages accrue to the practice of the present process when the membrane employed is formed as hollow fibers. Separatory devices employing hollow fibers and methods of making such fibers are now well known. See, for example, US. Pats. 3,228,876 (Mahon); 3,423,491 (McLain et al.); 3,536,611 (Filippi et al.); 3,503,515 (Tomsic) and 3,645,890 (Lukach et al.). See also U.S. Pats. 3,494,780 and 3,532,527 (Skiens), which teach methods of making hollow fibers of enhanced permeability. Typical dimensions of hollow fibers made by the methods of the preceding patents are from 50 to 300 microns ID. with Wall thicknesses of from 5 to 50 microns.

In the practice of the present invention, negligible transrnembrane pressure differentials will exist and accordingly, wall thicknesses as low as 5 microns are operable.

When the membrane is in the form of hollow fibers, the elfect of extractant solution viscosity becomes particularly important. The proportion and type of diluent in this case should be such that the viscosity of the extractant solution is not in excess of cps. Also, processing of efiluents containing suspended matter will generally not be practicable.

Suitable organic liquids for the practice of the present invention do not detrimentally affect the membrane used and comprise at least 1 weight percent of one or more polar extractants selected from groups (a) through (g) in the preceding list. These extractants have been found satisfactory for the recovery of lower carboxylic acids from quite dilute aqueous solutions. Those extractants which are liquids at the contemplated temperature of operation can be employed neat or diluted with an organic solvent which has little or no solubility in the aqueous acid solution to be processed. Those extractants which are solids-such as trioctyl phosphine oxide-will necessarily be dissolved in such a solvent.

The concentration of the extractant in the organic phase is primarily viscosity limited but may be as high as 100- percent.

Examples of suitable organic diluents are kerosene, carbon tetrachloride, amyl acetate, toluene, ethyl benzyl ketone, dibutyl ether, benzonitrile, 2,2-dichloro propane, heptane, l-nitrobutane, triethyl phosphite, l-pentanol, ethoxy (triethyl) silane and l-fluoropentane.

The choice of a diluent Will depend on several factors generally apparent to those skilled in the art. For example, the diluent must be a good enough solvent for the polar extractant or extractants to provide a total concentration of at least 1 weight percent. It must not dissolve the membrane or alter the membrane structure to such an extent that the effective pore diameter is put outside of the requisite 10 to 200 A. range. The diluent should not be significantly degraded by whatever physical or chemical treatment is used to strip the extracted acid from the loaded extractant solution. If stripping is done by contacting with an aqueous base, for example, the stripped extractant phase should be immiscible with and readily disengaged from the aqueous phase.

Any appropriate method may be used to remove the carboxylic acid from the loaded extractant or extractant solution. For example, the acid can be distilled off as an azeotrope with a portion of the diluent and then frozen. out, base precipitated as a salt or selectively adsorbed. Alternatively, the loaded extractant phase may Chemical analyses be directly contacted with gaseous ammonia and the resulting ammonium carboxylate removed by water washing or filtration. Stripping may also be accomplished by counter-current contacting with a member of volumes of water.

The transfer of acid from the aqueous feed to the extractant phase through the membrane can be carried out with either co-current or counter-current flow of two phases The latter mode ls more efliclent and 10 concentrations were also determined with a potentiowln generally be preferred metric titrator with initially one part tributyl phosphate,

SPECIFIC EMBODIMENTS three parts acetone and one part Water. The resulting tWohase s stem was tritrated at no reater rate than Dialysis studies with simulated phenol plant effluent 20 per Iginute of 10 N KOH withgvigomus Stirring.

brine were conducted with the Dow hollow fiber kidney, Tribut 1 hos was obtained from the and the National Bureau of Standards (NBS) dialysis garlic chgmgcalsrbivisiomFMc corporation and was used Chloride ion-Chloride was determined in aqueous media by coulombmetric titration using an Aminco-Cotlove instrument built by the American Instrument company.

Acetic acid.Both organic and aqueous streams were analyzed for acetic acid. Aqueous concentrations were determined with a potentiometric titrator; no further sample preparation was necessary. Organic acetic acid test cell H u fib as received. Trace TBP in the brine phase was determined 0 OW er 1 Hey by back extraction into CS and gas chromatographic Several experiments were performed with kidney analysls of the Organic 2 l1 t units to determine non-aqueous single pass, counterconcentratlons were dethl'mlhed y current mass transfer coefficients. The Dow kidney conultravlolet absorPtlfm M- Tesfs 0f mblltyl P tains about one square meter of surface area of regen- P P saturated brme mdlcated Interference y the erated cellulose fibers which are 210,0. I.D. X 260p. O.D. hrlne, TBP of a t teand resembles a miniature shell and tube heat exchanger. EXAMPLE 1 About 10,000 hollow fibers are closely spaced in parallel as a bundle about 1% inches in diameter and 6 inches Distribution of acetic acid between brine and tributyl long. The fiber ends pass in sealing arrangement through phosphate silicone rubber tube sheets and communicate With The olubility of TBP was determined to be 50 p,p.m inlet and outlet headers. The fiber/tube sheet assemi uilibrium with waste brine. With trioctylphosphine y is mounted in a Shell having inlet and Outlet oxide, the equilibrium solubility is about 1 p.p.m. in the nections. Generally, the metered organic stream was t b i P p through the inside of the fibers While the metered To evaluate the maximum possible extraction of acetic aqueous Waste Stream was P p through the Shell acid into TBP, the distribution ratio was determined at around the Outside of e fibers- Aqueous e s were acid concentrations of 1500, 5000 and 10,000 p.p.m. The substituted for the organic to study the contribution of Synthetic b i contained 20% di chloride and pH the organic phase to the overall mass transfer resistance. adjustments to 30 were made with hydrochloric acid A In all cases the feed 'F waste brme Streams distribution ratio of 5 in favor of the organic phase was were assayed for acetic acid content. Several samples found at each concentration were taken during each run to assure a steady state 40 situation in the contractor. The overall mass transfer coefficient was calculated using a 1n mean concentration The mass transfer data obtained with the hollow fiber driving force. kidney is summarized in Table I.

Hollow fiber kidney-overall mass transfer coeflicients TABLE I.-MASS TRANSFER COEFFICIENTS-HOLLOW FIBER KIDNEY a Brine feed Dialysate Mass Transfer Flow Flow transferred, coefiirate, rate, grams per cient Temp., 0. Composition cc./min. Composition cc./min. mJ/minute emJmin.

25 0.9% NaOl b 200 H20 500 1. 35 v 0. 023 65 0.9% NaCl 200 H20 500 1 u @025 25 20% NaCl, 0.2% acetic acid.-. 200 20% NaCl 500 0.28 d 0. 021 25 do 500 TBP 250 0 19 d 0. 016

B All measurements on same kidney. b Through inside of fibers: NaCl. 6 Acetic acid! NBS dialysis test cell Substitution of TBP for the 20% NaCl dialysate resulted in a 24% decrease in the measured mass transport Lang and Stokesherry of the National Bureau of coefficient which probably results from the increased Standards have developed a test cell to measure the dif- 6O viscosity f th TBP, i 'fi fl afi fl h Solute dif. fusive transport across a semipermeable membrane. A f ion rate, membrane area of 51 cm. is used with this dialyzer. The liquid phase transport resistances are minimized through the inclusion of turbulence promoting screens The transfer coefficients obtained with the NBS cell and narrow flow channels on each side of the membrane. at 25 C. are summarized in Table II. Aqueous/non-aqueous dialysis studies with this type cell TABLE TRANSFER COEFFICIENTS NBS TEST were performed to estimate the transport potential if CELL NBS test celloverall mass transfer coeificients only the membrane or fibers impeded dialysis of acetic Transfer acid from the aqueous to the organic. In the NBS test coefficient cell the two streams were circulated from closed systems Brine Dialysat" cm'lmm at high velocity through the test cell on each side of 0.9% NaCl H2O 1 the membrane. The changes in reservoir concentrations $8 were followed with time. All flat membranes used with =0. 02

this cell were 1.0 mil thick Bemberg type cellophane With the NBS dialyzer a 30% reduction in mass transfer coefiicient accompanied the substitution of the TBP for the 20% NaCl dialysate. The measured transfer coeflicient of 0.027 cm./min. for the TBP system more fully reflects the permeability achievable in an industrial dialyzer. Here clinical constraints need not be imposed on the design as in the case of an artificial kidney.

EXAMPLE 2 Removal of monochloroacetic acid from 2% solution in water, using a hollow fiber beaker dialyzer The dialyzer used was of a type commercially available from The Dow Chemical Company. A tow of one thousand fibers, about 7" long, hangs as a U-shaped loop having its terminal portions potted in inlet and outlet coupling members integrally formed with the closed end of an inverted plastic beaker about 2 in diameter and 3" high. Inlet and outlet connections are joined to the coupling members to provide for passage of the solution being treated through the fiber lumen. Dialysate inlet and outlet connections are also formed integrally with the closed end of the beaker. The open (lower) end of the beaker is provided with a recessed flange, which retains a plastic screen disk, and is joined in a fluid tight seal to a shallow plastic cup in which a mangetic stirrer bar is placed. The

fibers are of cellulose and have inner diameters of 200 microns, outer diameters of 250 microns and a total elfective membrane area of 1000 cm.

(A) 75 ml. of a 10 wt. percent solution of trioctyl phosphine oxide in kerosene was placed in the beaker and 100 ml. of the 2% (21.2 g./l.) chloroacetic acid solution was circulated through the fiber interiors at a rate of about 100 ml. a minute. A magnetic stirrer was employed to agitate the organic phase. After 20 minutes the concentrations of the acid in the organic and aqueous phases were respectively 17.9 and 7.9 grams/ liter. After four hours equilibrium had been attained and the organic and aqueous concentrations were 23.0 and 4.0 grams/liter, respectively. The distribution coefiicient (D) was then 5.75 and the mass transfer coefficient was calculated as:

K O A l. an 1 om The temperature during the test was 25 C. (B) The dialysis of preceding test A was repeated, but the organic phase consisted 100% of tetra-n-butyl urea.

Chloroacetic acid concentration, g./l.

Organic Aqueous Dialysis time phase phase 70 min 17.0 8.45 4 hrs. (equilibrium)- 22. 4. 9

8 The distribution coefiicient was 4.5 and the mass transfer coefiicient, calculated as above, was 0.002 cm./min.

Note: The viscosity of undiluted tributyl urea is about cps. and this presumably accounted for the relatively low transfer coefiicient.

Electron microscope examination shows that in each of the membranes employed in the preceding example, the average effective pore diameters are Within the range of 10 to 200 A.

We claim:

1. A process for the recovery of lower carboxylic acids from dilute aqueous solution, comprising:

contacting one side of a microporous membrane with said aqueous solution while contacting the other side of the membrane with a water immiscible, liquid organic solvent for said acid,

said membrane having an average effective pore diameter of from 10 to 200 A and being not detrimentally affected by said aqueous solution or said organic solvent,

said aqueous solution having a lower carboxylic acid content of two percent or less by weight, said content consisting of one or more two to four carbon, monoor dicarboxylic acids substituted with from zero to three chlorines or fluorines, or with zero or one hydroxyl, oxo, cyano, methoxyl or ethoxyl group,

the carbon to oxygen atomic ratio in each of said acids being not less than 1.0 nor greater than 2.0,

said organic solvent comprising a total of at least one weight percent of one or more,

and separating the resultant loaded extractant and raffinate solutions.

2. The process of claim 1 in which said membrane is in the form of hollow fibers.

3. The process of claim 1 in which said extractant is trioctyl phosphine oxide.

4. The process of claim 1 in which said aqueous solution is a brine containing about 18-20 wt. percent sodium chloride and about .2 wt. percent sodium acetate which has been treated with an amount of hydrochloric acid equivalent to the amount of sodium acetate present.

VIVIAN GARNER, Primary Examiner US. Cl. X.R.

260-465.4, 485 S, 526 N, 52 6 R, 535 R, 539 A, 539 R, 540, 541

- Po-wso Ufi'ITED S' I ATES PATENT OFFICE v v v v CERTIFICATE OF'CORRECTION v Patent 3.808.267 Dated April 30, 197k Inventofls) Jr C. Davis andR. R. Grinstead' It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 5, linehl "Contractor" should be contactor Column 8, line 31 should read weight percent of one or more trialkyl phosphine oxides wherein each R is a. C to C alkyl and the total number of carbons in the molecule is from 10 to 36 Signed and sealed this 26th day of November 1974.

(SEAL) Attest: v

McCOY M. GlBSON JR. c, MARSHALL DANN Attesting Officer Commissioner of Patents 

